The invention relates to a pressure switch for monitoring tire pressure of motor vehicle tires, which is fastened at a wheel rim of a vehicle tire and is actuated by inflating pressure in the tire. The switch comprises a reference pressure chamber which is sealed off by an electrically conductive switching diaphragm facing in the direction of the tire. The chamber carries a contact pin interacting with this switching diaphragm, to close an electrical circuit in response to tire pressure and vehicle speed.
A tire pressure sensor for monitoring tire pressure is known from German Patent Application No. DE-OS 35 43 864, and comprises a pressure switch with a reference pressure chamber. The switch is sealed off in the direction of the tire interior, by an electrically conductive switching diaphragm and includes a contact pin which interacts with this diaphragm. When the pin is in contact with the switching diaphragm, a specified operating pressure has been reached in the vehicle tire. Such a pressure switch can be designed for only one switching pressure actuation Point, regardless of the speed range of the vehicle; i.e., the single pressure switch can monitor only one inflating pressure either at high or low vehicle speeds.
It is the object of the invention to provide a pressure switch for monitoring tire pressure of motor vehicle tires, which is fastened at a wheel rim of a vehicle tire and is actuated by inflating pressure in the tire. The switch comprises a reference pressure chamber which is sealed off by an electrically conductive switching diaphragm facing in the direction of the tire. The chamber carries a contact pin interacting with this switching diaphragm, to close an electrical circuit in response to tire pressure and vehicle speed. Thus, this switch can set a minimum inflating pressure of a vehicle wheel to be monitored as a function of the vehicle speed.
This object, according to the invention, is obtained by having the switching diaphragm have a mass which is controlled as a function of the centrifugal force of the wheel such that the switching point of the pressure switch can be changed automatically. The switching points of the pressure switch form a characteristic curve as a function of the speed of the vehicle which sets a warning threshold for an excessively low tire pressure according to vehicle speed. The switching diaphragm of the pressure switch is arranged in a vertical transverse plane which extends approximately at a right angle with respect to the direction of force of the centrifugal force.
In order to vary the relationship with speed, the switching diaphragm has an accumulation of mass opposite the switching pin. This accumulation of mass can be located in a centric portion of the diaphragm; can be distributed uniformly over the circumference of the diaphragm; can consist of a thickened wall of the switching diaphragm; or can consist of a centrically arranged sleeve which is connected with the switching diaphragm and surrounds the free end of the contact pin.
The main advantages achieved by the invention consist of the fact that, as a result of the special construction of the switching diaphragm, the switching point of the pressure switch can be adapted to the minimum inflating pressure required for a given speed, by use of an accumulation of mass at the switching diaphragm whereby the enlarged surface mass produces a different switching over point in response to centrifugal force which increases with the vehicle speed. This centrifugal force causes a corresponding deflection movement of the switching diaphragm which results in an automatically changeable switching point as a function of the speed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.